A boiling reactor water (hereinafter referred to as BWR) type nuclear power plant compulsorily circulates cooling water by a recycling pump or an internal pump in the reactor which contains fuel rods in a pressure vessel in order to efficiently transfer heat generated in fuels to cooling water. The cooling water vapor generated in the reactor is mostly used to drive a steam turbine for power generation. The vapor which is discharged from the turbine is condensed by a condenser, and the condensate is almost completely degassed to be recycled back to the reactor as cooling water. The almost completely degassing removes oxygen and hydrogen produced by radiolysis of water in the reactor core. The condensate is treated, before being recycled back to the reactor, by using an ion-exchanging resin filter, e.g. desalter, so as to mainly remove metallic impurities to suppress formation of radioactive corrosion products in the reactor and heated to around 200° C.
Since the radioactive corrosion products occur at the portions where the pressure vessel, recycling system and the like contact with the reactor water, the corrosion-resistant stainless steel or the impermeable steel, e.g. nickel-based alloy, is used for the major reactor components in the primary system. Moreover, the reactor pressure vessel, which is made of a low-alloy steel, is treated with the weld overlay of stainless steel in order to prevent the low-alloy steel from coming into direct contact with the reactor water. In addition to these material considerations, one part of the reactor water is cleaned by a reactor water cleaning unit to positively remove metallic impurities present in water of which the concentration is very low.
However, the presence of metallic impurities in reactor water is unavoidable, and one part of these impurities will be deposited in the form of metal oxides on the fuel rod surfaces. The metals deposited on the rod surfaces are subjected to nuclear reactions by the irradiation of neutrons from the fuels to become radionuclides, e.g. cobalt 60 or 58, chromium 51, or manganese 54. Most of these radionuclides remain deposited on the rod surfaces in the form of oxides. However, one part of them is eluted into cooling water while depending on the solubility of the oxide in which they are included, or re-released into the reactor water as the insoluble solid which is referred to as crud. The radioactive substances present in the reactor water are removed by a cleaning system. However, the unremoved ones will be deposited on the portions where the recycling system component contacts with the reactor water while the reactor water is circulating in the systems, e.g. recycling system. As a result, the radioactive ray may be radiated from the contaminated surfaces to cause radiation exposure of operators when conducting the regular inspection. A radiation dose to which operators may be exposed is controlled for individual operators not to exceed a stipulated level. This level was reduced to the lower level recently, and thus it is necessary to economically minimize the exposed dose of each operator as far as possible.
Therefore, the various measures have been studied to control deposition of radionuclides on piping systems and to reduce radionuclide concentrations in reactor water. For example, Patent Document 1 proposes to inject metallic ion, e.g., zinc ion, into reactor water to form a densely, zinc-containing oxide film on the surfaces of the pipes in contact with the reactor water in a recycling system to prevent radionuclides, e.g, cobalt 60 or 58, from entering the film. Patent Document 2 proposes to coat the inner of the pipes in the recycling and cleaning systems, through which the reactor water flows while the plant is in service, with the oxide film under the certain conditions before cooling water is contaminated with the eluted or the released radionuclides.
Patent Document 1: JP-A-58-79196
Patent Document 2: JP-A-62-95498
However, injection of a metallic ion, e.g. zinc ion as disclosed by Patent Document 1, involves problems that the continuous injection of the ion is required while the plant is in service, and also the use of zinc obtained by isotopic separation is required in order to avoid radioactivation of zinc itself.
With reference to the method for coating the surfaces of the pipes with the oxide film, as disclosed by Patent Document 2, the inventors have found that there exist the following problems resulting from formation of the film at BWR operating temperature (250 to 300° C.). In short, the inventors of the present invention have found, when a reactor component of stainless steel is to be coated, that a Cr-rich inner layer of the oxide film is first formed on the surface and then an outer layer of the oxide film containing Cr with the lower content is formed. And the inventors have found that this two-layers structure has little effect of suppressing the radionuclide deposition, because radioactive cobalt 60 or 58 easily enters the inner layer of the oxide film.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.